The synchronous optical network (SONET) standards for optical transmission include a number of mechanisms and recommendations for protecting transmitted traffic. These include making use of “rings” that contain all of the transmission termination points. Such rings provide two paths between each transmission point on the ring, thus providing a level of redundancy. There are two basic ways to exploit SONET rings to protect traffic: a uni-directional path switched ring (UPSR) and a bi-directional line switched ring (BLSR).
A UPSR operates by having two copies of the same traffic go in opposite directions from one side of a SONET ring to the other. At the exit point from the ring, the traffic from the two directions is compared, and the better version is selected for output. This approach can be utilized for asynchronous transfer mode (ATM) traffic using virtual path (VP) comparison. Specifically, the overhead associated with each ATM virtual path that provides performance indications is inspected to determine which path exhibits better performance by whatever performance criteria is chosen. ATM virtual paths include ATM cell streams that share the same 8-bit or 12-bit virtual path identifier (VPI) value in their cell headers.
Traditional approaches to UPSR for ATM involve switch reconfiguration. Specifically, an ATM switch is provided at the ring node where the VP is being extracted from the ring. This ATM switch is configured to switch the ATM traffic from one of the two VP copies through to a destination. The other VP copy is ignored by the ATM switch—it is not switched through to an outgoing transmission but is instead discarded. When the VP performance measures indicate that the VP copy being discarded has become the superior or preferred version of the two VP copies, the ATM switch is reconfigured to switch through this VP copy and to ignore the original VP copy. Such reconfigurations can be carried out indefinitely.
This type of ATM switch reconfiguration approach severely taxes the processing resources, such as a microprocessor (or other computing entity) that is controlling the switch. Reconfiguration is often a computationally involved task. The SONET standards require that switch reconfiguration occur within 50 ms of the failure of a transmission link that is carrying the active VP copy. This standard may not always be met under certain conditions.